This invention is generally in the area of the combinatorial chemistry, in particular, the use of combinatorial chemistry to optimize the Fischer-Tropsch synthesis of hydrocarbons in the distillate fuel and/or lube base oil ranges.
The majority of fuel today is derived from crude oil. Crude oil is in limited supply, and fuel derived from crude oil tends to include nitrogen-containing compounds and sulfur-containing compounds, which are believed to cause environmental problems such as acid rain.
Although natural gas includes some nitrogen- and sulfur-containing compounds, methane can be readily isolated in relatively pure form from natural gas using known techniques. Many processes have been developed which can produce fuel compositions from methane. Most of these process involve the initial conversion of methane to synthesis gas (xe2x80x9csyngasxe2x80x9d).
Fischer-Tropsch chemistry is typically used to convert the syngas to a product stream that includes combustible fuel, among other products. A limitation associated with Fischer-Tropsch chemistry is that it tends to produce a broad spectrum of products, ranging from methane to wax. Product slates for syngas conversion over Fischer-Tropsch catalysts (Fe, Co and Ru) are controlled by polymerization kinetics with fairly constant chain growth probabilities, which fix the possible product distributions. Heavy products with a relatively high selectivity for wax are produced when chain growth probabilities are high. Methane is produced with high selectivity when chain growth probabilities are low.
Methane can be recirculated to ultimately yield combustible liquid fuel. Wax can be processed, for example, by hydrocracking and/or hydrotreating followed by oligomerization, to yield combustible liquid fuel. However, it would be advantageous to have new methods for providing a product stream from a Fischer-Tropsch process that has a higher proportion of combustible liquid fuel with less methane to recirculate and/or less wax to process.
Traditional Fischer-Tropsch synthesis has been modified by incorporating an acidic component, such as a relatively acidic zeolite, into the catalyst bed. When C4+ alpha-olefins are produced, the alpha-olefins isomerize to more substituted olefins in the presence of the acid catalyst and/or form aromatics. This reduces the chain growth probability for C4+ and largely minimizes wax formation. For example, U.S. Pat. No. 4,086,262 to Chang et al. teaches conducting Fischer-Tropsch synthesis with ZSM-5 intimately mixed with the Fischer-Tropsch catalyst. Chang focused on obtaining high octane gasoline (i.e., highly branched hydrocarbons in the gasoline range).
Most work since then has focused on improving the catalyst components and continues to provide highly branched hydrocarbons in the high octane gasoline range. The catalysts are typically iron catalysts, since they operate at higher temperatures where the zeolites tend to be more active. In addition to intimate mixtures of zeolites and Fischer-Tropsch catalysts, some carbon monoxide hydrogenation components have been incorporated directly on zeolites (see, for example, U.S. Pat. No. 4,294,725).
There is a growing interest in developing xe2x80x9cgreenerxe2x80x9d diesel fuels, i.e., fuels that do not contain aromatic, nitrogen or sulfur compounds. Straight chain or slightly branched paraffins in the diesel fuel range tend to have relatively high cetane values. Ideally, such fuels could be provided directly from Fischer-Tropsch reactors, if the right combinations of Fischer-Tropsch catalysts and zeolites could be found. However, known combinations of zeolites and Fischer-Tropsch catalysts to date have provided mainly highly branched paraffins in the gasoline range.
It would be advantageous to provide new catalysts compositions for converting syngas to higher molecular weight products, for example hydrocarbons in the distillate fuel and/or lube base stock base oil ranges. The present invention provides such compositions.
The present invention is directed to methods for converting syngas to hydrocarbons in the distillate fuel and/or lube base oil ranges via Fischer-Tropsch synthesis. In one embodiment, the methods use cobalt/ruthenium Fischer-Tropsch catalysts in combination with an olefin isomerization catalyst, for example a relatively acidic zeolite, for isomerizing double bonds in C4+ olefins as they are formed. The composite catalysts described herein permit the Fischer-Tropsch synthesis to operate with relatively high chain growth probabilities through about C3, and with relatively low chain growth probabilities above C4.
In another embodiment, the methods use Fischer-Tropsch catalysts that may or may not be cobalt/ruthenium catalysts, in combination with catalysts that are acidic enough to isomerize the double bonds in C4+ olefins, yet not so strongly acidic that they coke rapidly. Preferably, the catalysts are zeolites with silica/alumina ratios of between 3 and 100. An additional benefit of using the relatively less acidic zeolites is that the ratio of iso-paraffins to aromatics may be increased relative to when more acidic zeolites are used.
The methods can advantageously be optimized using combinatorial chemistry, in which a database of combinations of catalyst systems and, optionally, catalyst pre-treatments and/or reaction conditions, which provide various product streams, are generated. As market conditions vary and/or product requirements change, conditions suitable for forming desired products can be identified with little or no downtime.
In this embodiment, libraries of catalysts suitable for use in a first catalyst system (Fischer-Tropsch catalysts) and a second catalyst system (olefin isomerization catalysts) are prepared. The libraries can optionally include catalysts that possess both types of activity, namely, that can convert syngas to olefins and also that isomerize the olefins.
The catalysts are preferably combined in a logical manner, for example in an Axc3x97B array, where each position in the A column includes one or more catalysts from the first catalyst system, and each position in the B row includes one or more catalysts from the second catalyst system. In this manner, virtually every possible combination of catalysts in the libraries can be evaluated. The combinations of catalysts can be evaluated using varied reaction conditions, which can provide a) a combinatorial library of product streams and a database including the combination of catalysts and reaction conditions to provide each product stream and/or b) the optimum combination of catalysts and reaction conditions for obtaining a desired product stream.
The products include olefins such as ethylene, normal paraffins, iso-paraffins, and combinations thereof, and preferably include iso-paraffins in the distillate fuel and/or lube base stock ranges, and, more preferably, iso-paraffins in the jet or diesel range.